1. Field of the Invention
The invention relates to an arrangement for hydraulically operating a rear cover, covering top or the like on a vehicle, with at least one double-acting hydraulic working cylinder, which, on the one hand, is connected to the vehicle and, on the other hand, to the covering top; and its working chambers are in connection with each one side of a switched pressure source via one check valve each, wherein both of the check valves have an mutually releasing device and each of the working chambers parallel with respect to the check valves is kept via a pre-stress valve, which steers into the tank, at a specified pressure level.
2. The Prior Art
Arrangements of the noted type are known and make possible, for example, the automatic operation of vehicle doors, engine hoods, maintenance caps or also of covering tops of trunks or a chamber holding a convertible top. By means of the releasable check valves, it is ensured that, in the case that the operational pressure is turned off or fails, the covering top remains hydraulically blocked in the respective position, in order to switch off uncontrolled movements and thus possibly resulting dangers. The pre-stress valve makes possible the further controlled movement of the covering top in this condition, for example, by hand, to allow for an emergency closure against the force specifed by the pre-stress valve.
A disadvantage of the described known arrangement of the mentioned type is especially the condition that the described emergency operation is only possible in connection with a volume-balanced working cylinder, since otherwise there are problems with the removal or supply of the hydraulic medium to the working chamber on the side of the piston or valve stem. A simple redirection of the excess volume pushed out of the individual working chamber into the tank would still be possible; however, there are problems with the pressure-less re-supply of hydraulic medium to the other working chambers in the case of an emergency, since for the use in vehicle, on the one hand, very thin, flexible lines have to used which, on the other hand, due to the tight accommodations, are relatively long, so that the after-suctioning working chamber is at least partially filled with fumigated air. This then results in that, after the release of the manually-operated covering top in an emergency, these fall back undefined and can cause damage and injury.
It is the object of the present invention to improve a known arrangement of the initially described type such that the described disadvantages can be avoided and especially that, in a simple and safe way, an emergency operation of the covering top is made possible also in the common working cylinders having different working chamber volumes on the side of the piston and valve stem.